This article provides a systematic review of epidemiological surveys of autistic disorder and pervasive developmental disorders (PDDs) worldwide, aiming to assess the impact of geographic, cultural/ethnic, and socioeconomic factors on prevalence estimates and clinical presentation. The median prevalence estimate for autism spectrum disorders (ASDs) was found to be 62 per 10,000, with variability in estimates across regions. The evidence does not support significant differences in PDD prevalence by geographic region or strong impacts of ethnic/cultural or socioeconomic factors. However, the power to detect such effects is limited in existing data sets, particularly in low-income countries. The increase in prevalence estimates over time is likely due to broadening diagnostic concepts, increased service availability, and heightened awareness. The lack of evidence from most of the world's population highlights the need for further research and capacity building in low- and middle-income countries. The review also discusses methodological issues, including characterization of PDD, case identification, and case evaluation, and provides insights into clinical presentation in different regions.This article provides a systematic review of epidemiological surveys of autistic disorder and pervasive developmental disorders (PDDs) worldwide, aiming to assess the impact of geographic, cultural/ethnic, and socioeconomic factors on prevalence estimates and clinical presentation. The median prevalence estimate for autism spectrum disorders (ASDs) was found to be 62 per 10,000, with variability in estimates across regions. The evidence does not support significant differences in PDD prevalence by geographic region or strong impacts of ethnic/cultural or socioeconomic factors. However, the power to detect such effects is limited in existing data sets, particularly in low-income countries. The increase in prevalence estimates over time is likely due to broadening diagnostic concepts, increased service availability, and heightened awareness. The lack of evidence from most of the world's population highlights the need for further research and capacity building in low- and middle-income countries. The review also discusses methodological issues, including characterization of PDD, case identification, and case evaluation, and provides insights into clinical presentation in different regions.